The present invention relates to a method of evaluating an optical transmission path to be coupled to an optical device for optical communication.
To connect an optical transmission path to an optical device efficiently, use may be made of an optical fiber whose core is provided with a convex tip, as taught in, e.g., Japanese Patent Laid-Open Publication No. 5-224098 (Prior Art 1 hereinafter). In Prior Art 1, the end portion of an optical fiber is etched to bare a core and then heated to configure the core as a convex tip. The convex tip, playing the role of a lens, transforms the image magnification and thereby implements efficient coupling between the fiber and an optical device. Specifically, in Prior Art 1, the fiber whose core has the convex tip is located on the optical axis of a laser beam to issue from a semiconductor laser (optical device) and having a wavelength of 1.55 .mu.m, while adjoining the laser. The fiber is a single mode fiber having an outside diameter of 125 .mu.m, and a core whose diameter is 5 .mu.m. The convex tip protrudes from the end of the fiber by 10 .mu.m, and has a radius of curvature of about 5 .mu.m. The dopant content in the core sequentially decreases in a square distribution with respect to the distance from the center due to thermal diffusion occurred during fabrication. It follows that by etching the flat end of the fiber with an etching liquid whose etching speed decreases with an increase in dopant content, it is possible to provide the core of the fiber with the convex tip at low cost.
Japanese Patent Laid-Open Publication No. 1-224709 (Prior Art 2 hereinafter) discloses a fiber with a convex tip and a method of producing the same. In Prior Art 2, a transparent cylindrical rod-like member having a convex end is adhered to the end of an optical fiber. The distance between the plane of adhesion and the convex tip of the rod and the radius of curvature of the tip are selected such that light is output from the tip in the form of a parallel beam. This kind of scheme facilitates the alignment of the fiber and allows the distance between such fibers to be increased. Specifically, the rod-like member adhered to the end of the fiber allows a desired distance and radius of curvature and therefore a desired beam condition to be selected. Assume that the fiber with the convex tip and having a desired distance and radius of curvature is applied to a fiber type optical circuit. Then, because light is output in the form of a parallel beam, a coupling loss can be maintained extremely low even if the fiber is dislocated in the direction of its optical axis. Consequently, the fiber can be positioned extremely easily in the direction of the optical axis, allowing the distance between such fibers to be increased.
Japanese Utility Model Laid-Open Publication No. 57-39015 (Prior Art 3 hereinafter) proposes a single mode optical fiber with a convex tip implementing efficient coupling stably. The fiber of Prior Art 3 has most of its clad removed at at least one end portion, and has its core configured as a convex tip. With this scheme, it is possible to align the core and convex tip automatically, and to control the radius of curvature of the convex tip relatively desirably on the basis of, e.g., the etching time and heating conditions. This successfully enhances the yield of the coupling portions of single mode fibers.
In any case, evaluating the radius of curvature of a core protruding from the end of a fiber and having a convex tip is essential when it comes to the efficient optical coupling of the fiber with an optical device. Because image magnification depends on the radius of curvature of the convex tip of the core, an optical fiber or similar optical transmission path whose convex tip has a radius of curvature different from a designed radius cannot be coupled to an optical device with a desired efficiency.
However, the convex tip of the core has a radius of curvature which is generally as small as several microns to several ten microns. It is therefore difficult to directly measure the radius of curvature without any error. This prevents an optical fiber or similar optical transmission path whose convex tip has a desired radius of curvature from being selected, resulting in irregular optical coupling with an optical device. None of Prior Art 1 though Prior Art 3 teaches a method of evaluating the radius of curvature of the convex tip of the core protruding from end of the fiber.